1. Field of the Invention
The present invention relates to cationic resins and to their use in electrodeposition, and particularly to cationic resins and capped polyisocyanate curing agents and to their use in electrodeposition.
2. Brief Description of the Prior Art
The application of a coating by electrodeposition involves depositing a film-forming composition to an electrically conductive substrate under the influence of an applied electrical potential. Electrodeposition has gained prominence in the coatings industry because in comparison with nonelectrophoretic coating methods, electrodeposition provides higher paint utilization, outstanding corrosion resistance, and low environmental contamination. Early attempts at commercial electrodeposition processes used anionic electrodeposition, where the workpiece being coated served as the anode. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has become increasingly popular and today is the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles manufactured are given a primer coating by cationic electrodeposition.
To maximize corrosion resistance over steel substrates, cationic electrodeposition compositions are conventionally formulated with lead as either a pigment or as a soluble lead salt. In addition, the steel substrate is usually pretreated before electrodeposition with a phosphate conversion coating and rinsed with a chromic acid solution (chrome rinse) after pretreatment. However, lead and chromium are very toxic materials. The lead is often present in the effluent from electrodeposition processes and chromium is present in the effluent from pretreatment processes, and these metals need to be removed and disposed of safely, which may require expensive treatment processes.
It would be desirable to provide an electrodepositable composition having excellent corrosion resistance, which does not require lead nor the use of a chrome rinse.